The invention relates to a bone screw and an associated fastening tool for fixing the bone screw.
A bone screw is known, for example, from EP 0 483 242 B1. A bone screw is known from EP 0 614 649 A1. With these bone screws high torques are applied when the screws are driven home and also for fixing the screw element and for fixing the rod. As the screws have to be finally tightened during the operation by the person performing the operationxe2x80x94after inserting the rodxe2x80x94there is a danger of damaging the vertebra concerned. If the screws are not sufficiently finally tightened, there is a danger that the screws/rod connections will come loose and the bone screw will lose its fixed position.
It is the object of the invention to improve the way of fixing the bone screw, in order on the one hand to be able to apply sufficiently high torques for permanent fixing of the screws and on the other hand to reduce or avoid transfer of support moments on to the bone or the vertebra during screwing down.
By means of the screw connection in opposite directions both screw elements can be fixed at the same time. The respective support moment which occurs on tightening a screw element therein simultaneously serves as a torque supporting the screw connection of the other screw element. In this way higher torques can be applied without a dangerous torque being introduced into the bone of the patient.
With the tool according to the invention it is possible to apply a high torque in two opposite directions, so they almost cancel one another out. It is further possible for the user to vary the torque individually for the two screw elements in a simple way.
Further advantageous configurations of the invention are the subject of the subordinate claims.